A number of different stents have been proposed for the stenting of a blood vessel that has been occluded. A widely used type of stent consists of an expandable metal mesh. This type of stent may be further divided into self-expanding stents and non-self-expanding stents.
The self-expanding stents can be made of a mesh material that changes to a larger-size configuration upon heating to body temperature. Examples of stents of this type may be found in U.S. Pat. No. 6,071,308. Other self-expanding mesh stents are made of a resilient material, which can be flexed down into a small diameter tube and held in place in this configuration until it is released, at which time the mesh expands to the larger configuration.
The non-self-expanding stents are often expanded by use of an inflatable balloon, which is placed inside the mesh being in the small diameter configuration and which is then inflated, thereby expanding the mesh to the large diameter configuration. The balloon itself is then deflated for removal, while the metal mesh is left in the expanded configuration. For examples of non-self-expanding stents, see U.S. Pat. No. 5,799,384 and the international application WO-0189417.
Some of these expandable metal mesh stents are combined with an expandable polymer layer, which may be positioned on the inside of the expandable mesh, on the outside of the expandable mesh, within the interstices of the expandable mesh, or any combination of inside, outside and within the interstices of the expandable mesh stent. A stent of this type is, for example, shown in U.S. Pat. No. 5,968,070, wherein the polymer layer may consist of expanded polytetrafluoroethylene (PTFE). As disclosed in, for example, U.S. Pat. No. 5,160,341, it is also possible to use a polymer layer made of a resorbable polymer, such as polylactic acid homopolymers, polyglycolic acid homopolymers, or copolymers of polylactic acid and polyglycolic acid.
One advantage with expandable metal mesh stents is that their small diameter in the pre-expanded state allows easy insertion into narrow vessels. However, after the expansion, the metal mesh stents are difficult to remove since tissue in-growth occurs over time, and, in practise, the stents are normally left inside the blood vessel. The main complication associated with the stenting of a stenosis in a blood vessel is the risk of having a restenosis, in which case a new stenosis develops at the same position as the first one, i.e. a new stenosis is growing inside the inserted stent. Several types of stents have been suggested to handle this severe problem, including drug-delivering stents and radioactive stents. Examples of drug-delivering stents may be found in U.S. Pat. No. 6,206,195, while examples of stents for radiotherapy may be found in U.S. Pat. No. 6,192,271.
Nevertheless, there is still a substantial risk of having a restenosis following the stenting of a coronary artery. In this case, a second stent is normally inserted and expanded inside the first one, which obviously reduces the diameter of the second stent in its expanded configuration as well as the inner diameter of the re-stented blood vessel.
Further, when a stent is placed permanently inside a coronary artery, the continuous stress from the beating of the heart may cause the wall and edges of the stent to damage the vessel wall. This damage can lead to arterial rupture or aneurysm formation. Also, a stent adapted to be permanently implanted within a blood vessel is continuously exposed to the flow of blood inside the vessel, which may lead to thrombus formation within the blood vessel. Stents made of absorbable materials (see e.g. U.S. Pat. No. 5,306,286) have been proposed in order to overcome these problems. A disadvantage with such stents is that they are difficult to expand, i.e. they are of the self-expandable type. They have also a limited capability to withstand the compressive pressure exerted by the blood vessel in their expanded configuration.
A biodegradable polymeric stent having a programmed pattern of in vivo degradation is disclosed in U.S. Pat. No. 5,957,975. The stent comprises a substantially cylindrical element having two open ends and a plurality of different regions where each region has a desired in vivo lifetime.
And finally, U.S. Pat. No. 6,287,332 discloses an implantable, bioresorbable vessel wall support, in particular a coronary stent, that comprises a combination of metal materials which dissolves in the human body without any harmful effects on the person that wears the implant. The combination of metal materials can be an alloy or a local galvanic element. No specific structure of the stent is disclosed in U.S. Pat. No. 6,287,332.
It would therefore be desirable to provide a stent that combines the expandability and structural integrity of the metal mesh stents with the advantages of the absorbable stents. Such a stent would allow easy insertion into the blood vessel and yet being expandable enough to expand the blood vessel to the desired volume. The stent should also avoid the complications associated with permanently implanted stents by becoming dissolved or disintegrated. A stent having these characteristics would allow stenting of a restenosis, with the final inner diameter of the re-stented blood vessel being the same as after the first stenting operation.
Furthermore, a blood vessel provided with a stent in the longer term will loose some of its elasticity. An absorbable stent that relatively quickly and in a controllable manner looses its mechanical strength would enable, in an advantageous way, the blood vessel to rapidly regain its elasticity.